The semiconductor industry is characterized by a trend toward fabricating larger and more complex functions on a given semiconductor chip. The larger and more complex functions are achieved by reducing device sizes and spacing and by reducing the junction depth of regions formed in the semiconductor substrate. Among the feature sizes which are reduced in size are the width and spacing of interconnecting metal lines and the contact openings through which the metallization makes electrical contact to active device regions. As the feature sizes are reduced, new problems arise which must be solved in order to economically and reliably produce the semiconductor devices.
As both the contact size and junction depth are reduced, a new device metallization process is required to overcome the problems which are encountered. Historically, device interconnections have been made with aluminum or aluminum alloy metallization. Aluminum, however, presents problems with junction spiking which result from dissolution of silicon in the aluminum metallization and aluminum in the silicon. This problem is exacerbated with the small device sizes because the shallow junction is easily shorted and because the amount of silicon available to satisfy the solubility requirements of the aluminum metallization is only accessed through the small contact area, increasing the resultant depth of the spike. Adding silicon to the aluminum metallization has helped to solve this problem, but has, in turn, resulted in silicon precipitation and other problems.
One solution to this problem is to form a silicon diffusion barrier between the silicon substrate and the aluminum metallization. A conductive material such as titanium is deposited into the contact and annealed in a nitrogen ambient to form both a reactive silicide which directly contacts the device region and titanium nitride. An aluminum interconnect is then formed to contact the titanium nitride and the reactive silicide without direct contact by the aliminum to the active device region and without silicon precipitation in the contact. The formation of the titanium diffusion barrier in the contact requires that a layer of titanium be deposited on the surface of the insulating layers which overlie the active device region and through which the contacts are formed. Typically, the insulating layers include a doped glass layer which is applied to the substrate and annealed to form a planar surface prior to forming the interconnect metallization. A doped glass layer is preferred because the dopant causes the glass to flow at a sufficiently low temperature such that the previously formed shallow junctions will not be diffused further into the substrate.
The diffusion barrier metal must adhere well to the doped glass layer if the device metallization is to hold together when the individual die are separated from each other prior to packaging. During the process of die separation severe shear stress occurs which can result in the metal lifting from the die. Additionally, the interconnect metallization is stressed by compressive and pulling forces when package leads are bonded to the metal interconnects. In extreme cases the portion of the metal interconnect at the bond site is completely torn away from the die. Given that barrier metals comprising titanium and titanium alloys do not adhere well to doped glass, the increased use of barrier metals in advanced semiconductor devices has been accompanied by an increase in metal lifting and bonding failure.